Universal mobile telecommunications system (UMTS) is a 3rd generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3GPP LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
FIG. 1 shows network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an evolved packet core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNB) 20, and a plurality of user equipment (UE) 10. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways (S-GW) 30 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from eNB 20 to UE 10, and “uplink” refers to communication from the UE to an eNB. UE 10 refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNB 20 provides end points of a user plane and a control plane to the UE 10. MME/S-GW 30 provides an end point of a session and mobility management function for UE 10. The eNB and MME/S-GW may be connected via an S1 interface.
The eNB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNBs 20.
The MME provides various functions including non-access stratum (NAS) signaling to eNBs 20, NAS signaling security, access stratum (AS) security control, Inter core network (CN) node signaling for mobility between 3GPP access networks, Idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), packet data network (PDN) GW and serving GW selection, MME selection for handovers with MME change, serving GPRS support node (SGSN) selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for public warning system (PWS) (which includes earthquake and tsunami warning system (ETWS) and commercial mobile alert system (CMAS)) message transmission. The S-GW host provides assorted functions including per-user based packet filtering (by e.g. deep packet inspection), lawful interception, UE internet protocol (IP) address allocation, transport level packet marking in the downlink, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/S-GW 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.
A plurality of nodes may be connected between eNB 20 and gateway 30 via the S1 interface. The eNBs 20 may be connected to each other via an X2 interface and neighboring eNBs may have a meshed network structure that has the X2 interface.
FIG. 2 shows architecture of a typical E-UTRAN and a typical EPC.
As illustrated, eNB 20 may perform functions of selection for gateway 30, routing toward the gateway during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, SAE bearer control, and ciphering and integrity protection of NAS signaling.
FIG. 3 shows a user-plane protocol and a control-plane protocol stack for the E-UMTS.
FIG. 3(a) is block diagram depicting the user-plane protocol, and FIG. 3(b) is block diagram depicting the control-plane protocol. As illustrated, the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well known in the art of communication systems.
The physical layer, the L1, provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. Between different physical layers, namely, between physical layers of a transmission side and a reception side, data is transferred via the physical channel.
The MAC layer of the L2 provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of the L2 supports the transmission of data with reliability. It should be noted that the RLC layer illustrated in FIGS. 3(a) and 3(b) is depicted because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself is not required. A packet data convergence protocol (PDCP) layer of the L2 performs a header compression function that reduces unnecessary control information such that data being transmitted by employing IP packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the L3 is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). Here, the RB signifies a service provided by the L2 for data transmission between the terminal and the UTRAN.
As illustrated in FIG. 3(a), the RLC and MAC layers (terminated in an eNB 20 on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The PDCP layer (terminated in eNB 20 on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.
As illustrated in FIG. 3(b), the RLC and MAC layers (terminated in an eNodeB 20 on the network side) perform the same functions for the control plane. As illustrated, the RRC layer (terminated in an eNB 20 on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway 30 on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE 10.
The RRC state may be divided into two different states such as a RRC_IDLE and a RRC_CONNECTED. In RRC_IDLE state, the UE 10 may receive broadcasts of system information and paging information while the UE specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area and may perform PLMN selection and cell re-selection. Also, in RRC_IDLE state, no RRC context is stored in the eNB.
In RRC_CONNECTED state, the UE 10 has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the network (eNB) becomes possible. Also, the UE 10 can report channel quality information and feedback information to the eNB.
In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 10 belongs. Therefore, the network can transmit and/or receive data to/from UE 10, the network can control mobility (handover and inter-radio access technologies (RAT) cell change order to GSM EDGE radio access network (GERAN) with network assisted cell change (NACC)) of the UE, and the network can perform cell measurements for a neighboring cell.
In RRC_IDLE state, the UE 10 specifies the paging DRX cycle. Specifically, the UE 10 monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle.
The paging occasion is a time interval during which a paging signal is transmitted. The UE 10 has its own paging occasion.
A paging message is transmitted over all cells belonging to the same tracking area. If the UE 10 moves from one tracking area to another tracking area, the UE will send a tracking area update message to the network to update its location.
FIG. 4 shows an example of structure of a physical channel.
The physical channel transfers signaling and data between layer L1 of a UE and eNB. As illustrated in FIG. 4, the physical channel transfers the signaling and data with a radio resource, which consists of one or more sub-carriers in frequency and one more symbols in time.
One sub-frame, which is 1 ms in length, consists of several symbols. The particular symbol(s) of the sub-frame, such as the first symbol of the sub-frame, can be used for downlink control channel (PDCCH). PDCCHs carry dynamic allocated resources, such as PRBs and modulation and coding scheme (MCS).
A transport channel transfers signaling and data between the L1 and MAC layers. A physical channel is mapped to a transport channel.
Downlink transport channel types include a broadcast channel (BCH), a downlink shared channel (DL-SCH), a paging channel (PCH) and a multicast channel (MCH). The BCH is used for transmitting system information. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The PCH is used for paging a UE. The MCH is used for multicast or broadcast service transmission.
Uplink transport channel types include an uplink shared channel (UL-SCH) and random access channel(s) (RACH). The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell.
The MAC sublayer provides data transfer services on logical channels. A set of logical channel types is defined for different data transfer services offered by MAC. Each logical channel type is defined according to the type of information transferred.
Logical channels are generally classified into two groups. The two groups are control channels for the transfer of control plane information and traffic channels for the transfer of user plane information.
Control channels are used for transfer of control plane information only. The control channels provided by MAC include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network.
Traffic channels are used for the transfer of user plane information only. The traffic channels provided by MAC include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.
Uplink connections between logical channels and transport channels include a DCCH that can be mapped to UL-SCH, a DTCH that can be mapped to UL-SCH and a CCCH that can be mapped to UL-SCH. Downlink connections between logical channels and transport channels include a BCCH that can be mapped to BCH or DL-SCH, a PCCH that can be mapped to PCH, a DCCH that can be mapped to DL-SCH, and a DTCH that can be mapped to DL-SCH, a MCCH that can be mapped to MCH, and a MTCH that can be mapped to MCH.
The specification of a home eNB (HeNB) is currently ongoing in 3GPP LTE. It may be referred to Paragraph 4.6.1 of “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (release 9)” to 3GPP (3rd generation partnership project) TS 36.300 V10.2.0 (2010-12). The HeNB is a small base station designed for use in residential or small business environment. The HeNB may be a femto cell or a pico cell. The HeNB is short range about tens of meter, installed by the consumer for better indoor voice and data reception.
FIG. 5 shows logical architecture of an E-UTRAN HeNB.
Referring to FIG. 5, a HeNB 50 may be connected with an EPC 60 through an S1 interface. A HeNB gateway (55, HeNB GW) may be deployed between the HeNB 50 and the EPC 60 to allow the S1 interface and to scale to support a large number of HeNBs. The HeNB GW 55 serves as a concentrator for the C(control)-Plane, specifically the S1-MME interface. The S1-U interface from the HeNB 50 may be terminated at the HeNB GW 55, or a direct logical U(user)-Plane connection between HeNB 50 and S-GW 56 may be used. The S1 interface may be defined as the interface between the HeNB GW 55 and the core network, between the HeNB 50 and the HeNB GW 55, between the HeNB 50 and the core network, and between the eNB and the core network. Also, the HeNB GW 55 appears to the MME as an eNB. The HeNB GW 55 appears to the HeNB as an MME. The S1 interface between the HeNB 50 and the EPC 60 is the same whether the HeNB 50 is connected to the EPC 60 via a HeNB GW 55 or not.
FIG. 6 shows overall architecture with deployed HeNB GW.
Referring to FIG. 6, an E-UTRAN may include one or more eNB 60, one or more HeNB 70 and a HeNB GW 79. One or more E-UTRAN MME/S-GW 69 may be positioned at the end of the network and connected to an external network. The one or more eNB 60 may be connected to each other through the X2 interface. The one or more eNB 60 may be connected to the MME/S-GW 69 through the S1 interface. The HeNB GW 79 may be connected to the MME/S-GW 69 through the S1 interface. The one or more HeNB 70 may be connected to the HeNB GW 79 through the S1 interface or may be connected to the MME/S-GW 69 through the S1 interface. Or, the one or more HeNB 70 may be connected to the MME/S-GW 69 through the S5 interface.
The one or more HeNB 70 may be connected to each other through the X2 interface. In this case, the one or more HeNB 70 have same closed subscriber group (CSG) IDs. That is, the X2 interface may be established between the HeNBs having the same CSG IDs. If the X2 interface between HeNBs exists, a X2-based handover procedure may be performed through the X2 interface.
FIG. 7A and FIG. 7B show an example of an intra-MME/S-GW handover procedure.
In E-UTRAN, network-controlled UE-assisted handovers may be performed in RRC_CONNECTED state. Part of the handover command comes from the target eNB and is transparently forwarded to the UE by the source eNB. To prepare the handover procedure, the source eNB passes all necessary information to the target eNB (e.g. E-RAB attributes and RRC context). When a carrier aggregation (CA) is configured and to enable seconday cell (SCell) selection in the target eNB, the source eNB can provide in decreasing order of radio quality a list of the best cells. Both the source eNB and the UE keep some context (e.g. C-RNTI) to enable the return of the UE in case of handover procedure failure. The UE accesses the target cell via a random access channel (RACH) following a contention-free procedure using a dedicated RACH preamble or following a contention-based procedure if dedicated RACH preambles are not available. If the RACH procedure towards the target cell is not successful within a certain time, the UE initiates radio link failure recovery using the best cell.
The preparation and execution phase of the handover procedure is performed without evolved packet core (EPC) involvement. It means that preparation messages are directly exchanged between the eNBs. The release of the resources at the source side during the handover completion phase is triggered by the eNB.
First, the handover preparation procedure is described.
0. Area restriction information is provided. The UE context within the source eNB contains information regarding roaming restrictions which where provided either at connection establishment or at the last timing advance (TA) update.
1. The source eNB configures the UE measurement procedures according to the area restriction information, and transmits a measurement control message to the UE through L3 signaling. Measurements provided by the source eNB may assist the function controlling the UE's connection mobility. Meanwhile, the packet data is exchanged between the UE and the source eNB, or between the source eNB and the serving gateway.
2. The UE transmits measurement reports by the rules set by i.e. system information, specification etc to the source eNB through L3 signaling.
3. The source eNB makes handover decision based on the measurement reports and radio resource management (RRM) information.
4. The source eNB transmits a handover request message through L3 signaling to the target eNB passing necessary information to prepare the handover procedure at the target side. UE X2/UE S1 signaling references enable the target eNB to address the source eNB and the EPC. The evolved radio access bearer (E-RAB) context includes necessary radio network layer (RNL) and transport network layer (TNL) addressing information, and quality of service (QoS) profiles of the E-RABs.
In the case of a UE under an RN performing handover procedure, the handover request message is received by the DeNB, which reads the target cell ID from the message, finds the target eNB corresponding to the target cell ID, and forwards the X2 message toward the target eNB.
In the case of a UE performing handover procedure toward an RN, the handover request is received by the DeNB, which reads the target cell ID from the message, finds the target RN corresponding to the target cell ID, and forwards the X2 message toward the target RN.
5. The target eNB performs admission control. The admission control may be performed dependent on the received E-RAB QoS information to increase the likelihood of a successful handover, if the resources can be granted by target eNB. The target eNB configures the required resources according to the received E-RAB QoS information and reserves a C-RNTI and optionally a RACH preamble. The AS-configuration to be used in the target cell can either be specified independently (i.e. an “establishment”) or as a delta compared to the AS-configuration used in the source cell (i.e. a “reconfiguration”).
6. The target eNB transmits a handover request acknowledge message to the source eNB through L3 signaling, and prepares the handover. The handover request acknowledge message may include a transparent container to be sent to the UE as an RRC message to perform the handover. The transparent container may include a new C-RNTI, target eNB security algorithm identifiers for the selected security algorithms, a dedicated RACH preamble, and possibly some other parameters i.e. access parameters, SIBs, etc. The handover request acknowledge message may also include RNL/TNL information for the forwarding tunnels, if necessary. Meanwhile, as soon as the source eNB receives the handover request acknowledge message, or as soon as the transmission of the handover command is initiated in the downlink, data forwarding may be initiated.
7. The target eNB transmits an RRC connection reconfiguration message including mobility control information to perform the handover, to be sent by the source eNB to the UE. The source eNB performs the necessary integrity protection and ciphering of the message. The UE receives the RRC connection reconfiguration message with necessary parameters. The UE is commanded by the source eNB to perform the handover procedure. The UE does not need to delay the handover execution for delivering the hybrid automatic repeat request (HARQ)/automatic repeat request (ARQ) responses to the source eNB.
Hereafter, the handover execution procedure will be described.
The UE detaches from old cell and synchronizes to new cell. In addition, the source eNB delivers buffered and in-transit packets to the target eNB.
8. The source eNB transmits a serial number (SN) status transfer message to the target eNB to convey the uplink packet data convergence protocol (PDCP) SN receiver status and the downlink PDCP SN transmitter status of E-RABs for which PDCP status preservation applies. The uplink PDCP SN receiver status may include at least the PDCP SN of the first missing UL SDU and a bit map of the receive status of the out of sequence UL SDUs that the UE needs to retransmit in the target cell, if there are any such SDUs. The downlink PDCP SN transmitter status indicates the next PDCP SN that the target eNB shall assign to new SDUs, not having a PDCP SN yet. The source eNB may omit sending this message if none of the E-RABs of the UE shall be treated with PDCP status preservation.
9. After receiving the RRC connection reconfiguration message including the mobility control information, the UE performs synchronization to the target eNB and access the target cell via RACH. The access to the target cell via the RACH may be a contention-free procedure if a dedicated RACH preamble was indicated in the mobility control information. Or, the access to the target cell via RACH may be a contention-based procedure if no dedicated preamble was indicated. The UE derives target eNB specific keys and configures the selected security algorithms to be used in the target cell.
10. The target eNB responds to the synchronization of the UE with UL allocation and timing advance.
11. When the UE has successfully accessed the target cell, the UE transmits an RRC connection reconfiguration complete message (C-RNTI) to confirm the handover procedure, along with an uplink buffer status report, whenever possible, to the target eNB to indicate that the handover procedure is completed for the UE. The target eNB verifies the C-RNTI sent in the RRC connection reconfiguration complete message. The target eNB can now begin transmitting data to the UE. The packet data is exchanged between the UE and the target eNB.
Hereafter, the handover completion procedure will be described.
12. The target eNB transmits a path switch request message to MME to inform that the UE has changed cell.
13. The MME transmits a user plane update request message to a serving gateway (S-GW).
14. The S-GW switches the downlink data path to the target side. The S-GW transmits one or more end marker packets on the old path to the source eNB and then can release any U-plane/TNL resources towards the source eNB.
15. The S-GW transmits a user plane update response message to MME.
16. The MME transmits a path switch acknowledge message to the target eNB to confirm the path switch request message.
17. The target eNB transmits a UE context release message to the source eNB to inform success of the handover procedure and trigger the release of resources by the source eNB.
18. When the UE context release message is received, the source eNB can release radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue.
When an X2-based handover is used between HeNBs and when the source HeNB is connected to a HeNB GW, an S1 UE context release request message including an explicit GW context release indication is transmitted by the source HeNB, in order to indicate that the HeNB GW may release of all the resources related to the UE context.
The handover procedure may be performed between a HeNB and a macro eNB. That is, a UE may want to be handed over from the HeNB to the macro eNB, or from the macro eNB to the HeNB. When the handover procedure is performed between the HeNB and the macro eNB, more efficient way to release the UE context is required.